Maybe you’ve done this already, but you need to select which zones the volume control adjusts. Push and hold the black volume control until you see the three zones. Then push the button again to toggle through which zone(s) you want to adjust - 1, 2, 3 (3 is cockpit), or all three at the same time. You will see the zone icon displayed more boldly for the selected zone. Once you’ve selected the zone(s), turn the knob to adjust the volume. If you’ve done this and selected zone 3 and the cockpit speakers still don’t work, you might want to check the output connectors from the amplifier to the different zones.
Edit: These instructions are for operating the unit from the controller pad in the cabin, just behind the captain’s seat. This is the control I almost always use. The main amplifier screen is very similar but as I recall, the volume is displayed slightly differently.
John